Electrophysiological properties of embryonic luteinizing hormone~releasing hormone (LHRH) containing neurons were studied. Olfactory placodes from mouse embryos (E12.5) were dissected and cultured on glass coverslips for 2~3 weeks in defined medium. LHRH neurons emerged from the olfactory placode explants by day 6 in culture together with other olfactory neurons and non~neuronal cells. Electrophysiological recordings were carried out on unidentified neurons by employing whole~cell patch pipettes which contained various intracellular solutions and an additional fluroescent vital dye Lucifer Yellow. Both voltage~ and current~clamp techniques were employed. Following the electrophysiological recordings, the cells which had been labelled with Lucifer Yellow, were processed for immunocytochemical identification of LHRH. Fifteen neurons were positively identified as LHRH~containing neurons. These neurons displayed spontaneous spike discharges which were either generated intrinsically or transsynaptically. The somatic region of these cells expressed voltage~senstitive sodium current (INa), potassium currents (IA,IK), and GABAA~ receptors. Non~LHRH containing, glutamic acid decarboxylase (GAD), and GABA~containing neurons in these explant cultures were also studied. Electrophysiological properties of mouse fibroblasts (Balb~C3T3) transfected with cDNA encoding either gastrin~releasing peptide (GRP) or neuromedin B (NmB)~receptors were examined. Both receptors were expressed abundantly and upon activation of these receptors all the transfected cells responded with Ca2+ activated K+~conductance increases. Among various GRP~Bombesin receptor antagonists examined, [D~Phe6]~Bn(6~13) ethyl ester was the most effective in suppressing GRP~receptor activities. In general, the GRP antagonists were much less effective on NmB~receptors. Using a calcium imaging~photometry system, we have also shown that these peptides evoked intracellular Ca2+ increases in these transfected fibroblasts.